This process relates to a method for the selective hydrodealkylation and transalkylation of aromatic hydrocarbons and the cracking of paraffins. Alkyl aromatic compounds have long been produced from hydrocarbon fractions relatively rich in such materials. Early sources were liquids from coking or other distillation of coals. More recently, these products have been derived from fractions obtained in refining of petroleum. An important source in recent years has been the aromatic liquid naphthas known as pyrolysis gasoline resulting from the thermal cracking of gases, naphthas, and gas oils to produce olefins.
However derived, these aromatic-ring streams have usually been distilled and otherwise separated (e.g. solvent extraction) to obtain the desired product components. The purpose of these operations typically has been to obtain C.sub.8 aromatics and benzene which are now used in huge quantities in the manufacture of terephthalic acid and other chemical products. The separated streams usually resulting from the above separation by distillation or other means consist of product streams of benzene, toluene, C.sub.8 aromatics containing xylenes and a bottoms product of C.sub.9 and C.sub.10 + aromatics. The pyrolysis gasoline stream yields a C.sub.7 -C.sub.9 fraction which is usually blended into automotive gasoline.
This invention accordingly relates to a conversion process for the hydrodealkylation and transalkylation of fractionated pyrolysis gasoline containing ethylbenzene into more useful compounds. More specifically, this invention is concerned with a conversion process for the concurrent transalkylation and hydrodealkylation of a fractionated pyrolysis gasoline stream containing toluene, ethylbenzene and xylenes into ethylbenzene-lean xylenes and benzene utilizing a catalyst comprising a tungsten/molybdenum component of WO.sub.3 and MoO.sub.3 and an acidic catalyst of 60 (wt)% mordenite and 40 (wt)% catalytically active alumina.
The C.sub.8 aromatics found in pyrolysis gasoline generally contain approximately 44% ethylbenzene. The presence of ethylbenzene in mixed xylenes is detrimental to process yields and process economics when these xylenes are utilized in the production of p-xylene. Fractional distillation to remove ethylbenzene from mixed xylenes is not economically practical because of the closeness of their boiling points. Ethylbenzene can be removed from xylenes by repeated recrystallizations but this is economically very expensive and is technically difficult.
Pyrolysis gasoline is produced in an olefins plant by a steam cracking process which optimizes olefin production of carbon chain length of from C.sub.2 to C.sub.4. The composition of pyrolysis gasoline depends upon various factors, i.e., kind of feedstock, type of cracking unit and cracking conditions. All pyrolysis gasolines generally contain considerable amounts of aromatics, normally 40-80% benzene, toluene and xylenes, together with paraffins, naphthenes, olefins and diolefins. Pyrolysis gasoline is generally unstable and must be stabilized by selective hydrogenation immediately upon production in the olefins plant.
In the prior art, a pyrolysis gasoline stream from an olefins plant is hydrotreated with hydrogen to stabilize the stream and desulfurized to remove the sulfur. The stream is thereupon distilled in an aromatics recovery unit to remove the benzene, toluene and C.sub.8 -C.sub.9 fractions. The C.sub.8 -C.sub.9 fraction which includes C.sub.8 aromatics that contain 42-44% ethylbenzene are generally returned to the refinery to be used in gasoline blending at a lower economic value than the other products of the olefins plant.
In the prior art, a number of processes are peripheral to the instant invention of a process for producing benzene and ethylbenzene-lean xylenes from pyrolysis gasoline, but no processes are directly related to the instant invention.
There is no process utilizing hydrodealkylation and/or transalkylation in the prior art wherein toluene yields benzene and xylenes, ethylbenzene yields benzene, high boiling C.sub.7 -C.sub.9 paraffins and olefins yield C.sub.1 -C.sub.5 paraffins and xylenes are left unchanged simultaneously with a single catalyst, without side-reactions producing undesired products by hydrodemethylation thus destroying the desired products namely, the xylenes.
For example, U.S. Pat. No. 3,478,120 relates to a process and catalyst for the dealkylation of hydrocarbons which comprises contacting the said hydrocarbons with a catalyst comprising iron group metals and calcium aluminate. A stream comprising ethylbenzene and xylenes can be subjected to dealkylating conditions whereby the ethylbenzene and xylenes are selectively dealkylated to methane, ethane, toluene and benzene.
In another example, U.S. Pat. No. 3,919,339, discloses a process for selective hydrodealkylation of ethylbenzene to toluene and benzene in a hydrocarbon feedstock comprising ethylbenzene and xylenes, and for simultaneous isomerization of the xylenes to p-xylene. The U.S. Pat. No. 3,919,339 process comprises contacting the feedstock with a catalyst comprising a cobalt or nickel component on an acidic inorganic refractory oxide support at a temperature between 650.degree.-950.degree. F., a pressure below 300 psig and a hydrogen-to-hydrocarbon feed ratio between 1:1 and 20:1. Preferably, the catalyst used is cobalt on silica:alumina.
U.S. Pat. No. 3,957,621 discloses a method for the preparation of alkyl aromatic hydrocarbons by the processing of a fraction of heavy reformate, those from which benzene and lighter components have been largely removed by distillation. The fraction is hydrocracked over a zeolite catalyst associated with a hydrogenation/dehydrogenation component. Ethylbenzene is selectively removed from the charge stock which occurs in part by dealkylation of the side chain and in part by disproportionation to benzene and C.sub.9 + alkylbenzenes such as ethyltoluene and diethylbenzene instead of benzene and xylenes as in the instant application.
Alkyl aromatics such as toluene or ethylbenzene can be disproportionated to benzene and polymethylbenzenes, and benzene and polyethylbenzenes, respectively, as is disclosed in U.S. Pat. No. 3,578,723 by use of the crystalline zeolite catalyst ZSM-4. U.S. Pat. No. 3,578,723 does not teach dealkylation of ethylbenzene to benzene and ethane.
In another approach, ethylbenzene in a hydrocarbon stream can be dehydrogenated to styrene according to U.S. Pat. No. 3,720,726 using a zeolite catalyst having a mordenite crystal structure containing alumina fixed in combination therewith.
Accordingly, in the prior art there are a number of processes to dealkylate alkyl aromatics to benzene and polymethylbenzenes. Also, processes are in the prior art to dehydrogenate ethylbenzene to styrene. However, the prior art has not disclosed the process utilizing a catalyst composition comprising a tungsten/molybdenum component of WO.sub.3 and MoO.sub.3 and an acidic component of 60 (wt)% of mordenite and 40 (wt)% of catalytically active alumina whereby a heavy aromatics fraction of pyrolysis gasoline containing toluene, ethylbenzene, xylenes and sizeable amounts of non-aromatics selectively hydrodealkylated and transalkylated to ethylbenzene-lean xylenes and benzene.